When I read Coppola's comments that 'there will not be a GF IV', I took that as an opening where it won't really be a Godfather movie.

Let's be frank, the trilogy is about Michael. The flashback scenes in II were used to parallel Michael's rise, they weren't really about Vito.

It's feasible they could make a 20-30's period piece and call it The Don or Vito.

As far as DiCaprio, I thought he was thought to play Sonny, not Vito. If it's possible to make De Niro look 30ish without too much makeup then I'd go for that, but he's older now than Brando was in I.